Feed-forward techniques are finding increasing use in a variety of microwave communications systems. A particular application in which feed-forward amplifiers are especially useful is in multichannel amplitude modulated link (AML) transmitters for community antenna television (CATV) systems.
Typical feed-forward amplifier arrangements are designed to cancel distortion introduced by a main amplifier. Samples of the signal being processed are obtained both before and after the main amplifier and are compared to obtain an error signal. The error signal is amplified in an auxiliary amplifier and combined with the distortion-containing signal from the main amplifier such that cancelation of the distortion components occur. For further details concerning feed-forward microwave amplifiers, reference may be made to U.S. Pat. No. 2,592,716 to W. D. Lewis, U.S. Pat. Nos. 3,471,798 and 3,649,927 to H. Seidel, U.S. Pat. No. 3,886,470 to W. A. O'Neil et al., and to the publication by P. D. Lubell, "Linearizing Amplifiers for Multi-Signal Use", Microwaves, April 1974, pp. 46-50.
Applications such as CATV transmitters require that the feed-forward microwave amplifier arrangements be located outdoors; hence such arrangements may be subjected to wide variations of ambient temperature. Since the characteristics of the actual amplifying devices employed vary with temperature, the temperature range over which previous feed-forward microwave amplifier arrangements are able to provide effective distortion cancelation is limited.